A conventional headlamp lens is divided into many small prescription regions, or flutes. Certain of these flutes, referred to as spread flutes, are used for spreading out the headlamp beam. Such spread flutes have been used to provide foreground coverage of light to areas as far as 25 degrees to the side of the longitudinal axis of the vehicle. These spread flutes have typically included on their back surfaces alternating ridges and grooves, each extending vertically and each having the shape of a segment of a cylinder that is of uniform transverse cross-section at all points along its own central longitudinal axis. This uniform transverse cross-section may be circular, elliptical, sinusoidal, or any other shape, depending upon the light distribution desired from the ridge or groove. Flutes including such ridges and grooves of uniform transverse cross section are referred to herein as standard spread flutes.
While such headlamps provide a satisfactory pattern of light for those applications where the front of the lens is substantially vertical, a certain problem arises in those applications where the front of the lens is sloped back. More specifically, when the lens with the standard spread flutes is sloped back, the edges of the spread-out light beam emerging from such a flute droop, while the center of the light beam remains unaffected. Simply put, the standard spread flutes, when the lens is sloped back, do not spread the light as much, and the lost component ends up as down aim, or droop. (The term "sloped-back", as used herein, denotes that the bottom region of the lens is positioned ahead of the top region, considered in the direction of normal forward motion of the vehicle.)
The above-described problem is becoming increasingly more significant as car designers strive to improve the air flow over the front end of the car by specifying that the headlamp lenses be sloped back to match the front end curves of the car. The greater the slope, the more the beam tends to droop at its edges. Another factor sometimes present that can interfere with properly spreading the beam is the presence of a substantial rake angle in the orientation of the headlamp lens.